1. Field of the Disclosure
The present disclosure generally relates to photobleaching of luminescent materials and, more particularly, to a system and method of photobleaching using a fixed-wavelength, variable intensity laser illumination.
2. Brief Description of Related Art
Raman spectroscopy may be used for the identification of biological and chemical materials. A significant impediment to the acquisition of Raman spectra, particularly of samples of biological materials or industrial chemical mixtures/solutions, is the luminescence in the sample generated by the illumination used to generate the Raman scattering. This occurs when any or all of the material components being illuminated absorb light at the monochromatic wavelengths used to generate spontaneous Raman scattering. Once the material has achieved an excited electronic state through such absorption, emission (luminescence) over a broad range of wavelengths may occur. Because absorption and luminescence have a much higher overlap integral or cross-section (over the material surface) than does Raman scattering, the luminescence can often partially, if not totally, obscure the Raman scattered photons.
To overcome the problem of luminescence without resort to changing the excitation wavelengths, those skilled in the art have used a practice known as photobleaching. The method of photobleaching involves first bathing the sample in the laser light at the same wavelength and power to be used for the subsequent acquisition of Raman spectra. The principal result of this photobleaching is that the emission strength of the luminescence is significantly attenuated relative to that observed upon initial illumination. Furthermore, the emission strength of luminescence has been attenuated sufficient for the detection of Raman scattered photons. The duration of photobleaching required under these conditions can range from tens of minutes to several hours. Because of growing usage of Raman spectra in detection of biological and chemical threat agents in the field, it is desirable to expedite photobleaching and Raman detection processes so that decisions can be made quickly in the field (as opposed to a controlled, laboratory environment) without a long wait. It is therefore desirable to devise a photobleaching methodology that is significantly faster than the prior art approach.